Many convulsant agents enhance the responses of the brain to sensory stimuli. This enhancement occurs with subconvulsant doses and before any epileptiform event appears in the EEG, suggesting that it may reflect the mechanisms responsible for induction of seizures. Enhancement of evoked responses occurs earliest and is maximal in the brainstem reticular formation (RF). Investigations in this laboratory on the neuronal events subserving RF evoked response enhancement indicate that the responses of over 90% of RF neurons to sensory stimuli are significantly enhanced after convulsant administration. However, the responses of primary visual (lateral geniculate) neurons are minimally affected by convulsant, suggesting at least a relative specificity of this effect. Confirmation of these RF response changes in chronically prepared animals and extension of these investigations into other drug-induced, into kindling and into acute focal models of seizure will indicate the generality of the RF response change. The enhancement of sensory responsiveness in RF neurons has now been observed with 3 very different convulsants suggesting this effect may be a general action of convulsant agents. This action may reveal a general pathophysiologic brain mechanism involved in the triggering of seizures. The site of this action of convulsants will be investigated by examining the effects of reversible (cryogenic) lesions in primary sensory nuclei, changes in RF responses to electrical stimulation in these nuclei as well as in the cortex, and iontophoretic application of convulsants onto RF neurons. Results to date indicate that a major portion of the convulsant effect is exerted directly on the RF. The convulsant-induced neuronal response changes may have implications for other forms of seizure, since enhancement of sensory responses is also seen in several non-drug seizure models including certain forms of human epilepsy.